1. Field of the Invention
This invention relates to seismic data gathering devices or apparatuses. More particularly, it relates to testing the impedances and common mode rejections of geophone channels of such apparatuses. This invention is applicable to apparatuses for gathering seismic data on land, as well as to marine seismic data gathering apparatuses. However, for clarity the description herein will be directed primarily to land seismic data gathering apparatuses, with the understanding that the invention is not limited to land applications.
2. Description of the Prior Art
A land seismic data gathering apparatus comprises a plurality of geophone channels and a multi-channel signal recording and digital computing system which typically is mounted on a truck, and may be referred to simply as a recording and computing system. Each geophone channel includes one or more geophones and a plurality of wires connecting the geophones to the recording and computing system. Typically each channel includes two wires, which may be referred to as the high wire and low wire, respectively. Each wire has two ends, which may be referred to for convenience as the near end and the far end, respectively. With the recording and computing system as a reference point, the near end of each wire is connected to the recording and computing system and the geophones are connected to the wires at their far ends or at points between the two ends. The geophones detect vibrations in the earth and generate electric signals representative of such vibrations. The signals re transmitted by the wires to the recording and computing system for recordation and processing.
The accuracy and ease of interpretation of the seismic data gathered by the recording and computing system depends to a great extent on whether the impedances and common mode rejections of each of the geophone channels remain reasonably constant. Under normal conditions, the impendance of each geophone channel varies primarily with the length of its wires and with the condition of the geophones and their connections to the wires. The common mode rejection of each geophone channel varies primarily with the leakage from the wires to the earth. The leakage is to a great extent influenced by moisture on the earth. Therefore, the impedance and the common mode rejection of each geophone channel should be tested periodically between shots.
A commonly practiced method for making such impedance tests involves using complex switching means together with a resistance meter or ohmmeter. The geophone channels are tested seriatim by individually measuring the resistance across each channel with a resistance meter. The switching means are used to connect and disconnect each geophone channel to and from the recording and computing system and to and from the resistance meter according to the channel to be tested. See, for example, U.S. Pat. No. 2,917,706 (1959) to Thompson and FIG. 1 and the accompanying description below.
Other proposed geophone channel impedance testing methods employ a response test wherein switching means are used first for disconnecting the geophone channels from the recording and computing system and then for connecting the channels to a signal generator which simultaneously transmits a test signal to all the channels. Thereafter, the geophone channels are reconnected to the recording and computing system which simultaneously records the response signals of all the geophone channels. See, for example, U.S. Pat. No. 3,858,169 (1974) to Bardeen. See also U.S. Pat. No. 3,717,810 (1973) to Spanbauer which proposes driving the geophone channels with a constant RMS voltage or current and deriving the impedance from a measurement of the RMS value of the other of the voltage or current. Another proposed system involves impressing AC and DC currents of predetermined amplitudes on said geophone channels and detecting the excess of peak voltage produced by the AC current over the DC voltage generated by the DC current. See U.S. Pat. No. 4,052,694 (1977) to Fredriksson.
In the known prior art, the common mode rejection of the geophone channels has not been measured directly, but has been measured indirectly by use of an ammeter to measure leakage to the earth for each channel.
In a modern seismic data gathering apparatus up to several hundred geophone channels may be employed. Because all known prior art methods for measuring geophone channel impedance and common mode rejection involve at least one switch for each channel, therefore, a large number of switches are required. These switches are relatively expensive and are subject to failure under adverse field conditions. Further, the testing of the geophone channels seriatim, for those systems which require such a procedure, can be quite time consuming, particularly if one or more switches must be repaired. Such delays can be quite expensive, because the task of gathering seismic data is halted while such tests are being conducted.